When a joint between a bolt and a nut is tightened a clamp force is produced, which will keep the joint tightened. The clamp force is produced as a result of that a so called clamp length of the bolt is deformed, i.e. elongated. Generally, a tightening operation may be divided into two main phases; a first phase in which the bolt is threaded into the nut without deformation, and a second phase in which the joint between e.g. the bolt and a nut is established, and in which the bolt is being deformed. The bolt may be both elastically and plastically deformed. The transition point at which the threading phase ends and the deformation phase starts is conventionally denoted as snug.
In order to optimize a tightening operation it is desired to control the clamp force which is produced in the joint. It is however difficult to measure the clamp force directly, since that would imply having to place a force meter inside the joint, which is rarely possible. Instead, methods have been developed to measure the clamp force indirectly. One such method involves the use of ultra-sound to measure the elongation of the bolt. In other methods the joint is tightened to a certain torque instead of to a certain clamp force.
In the ultra sound method ultra-sonic pulses are transmitted into the bolt and the response time, often referred to as the time of flight, is monitored. The time of flight corresponds to the length of the bolt. Hence, any measured increase in the time of flight corresponds to an increase of the length of the bolt, and thus, of the clamp force in the bolt. The time of flight increases linearly with the elongation of the bolt, at least as long as the bolt is only elastically deformed. However, if the bolt is deformed to an extent that it starts to deform plastically, which is desired in certain applications, a new linearity will be established. Normally, the time of flight increases at a slower rate during the plastic deformation than during the elastic deformation.
A problem in the ultra sound method concerns the evaluation of the measured results. To start with, it is difficult to measure the time of flight in the bolt in a cost and time efficient manner before snug has been reached in the tightening operation. This is partly due to the fact that it is difficult to establish a good enough contact between the ultra-sonic meter and the bolt before the bolt has been clamped, e.g. before snug. After snug the bolt will be clamped in the joint and it will be much easier to achieve a good contact between the ultra-sonic meter and the bolt.
A conventional way of achieving a good contact is to incorporate a contact piece and an ultra-sonic pulse generator, e.g. in the form of a piezo-electric component in the head of the bolt. This is however a very costly solution and such a bolt will cost at least 20 times the price of an ordinary bolt.